A 68-year-old man with hypertension and diabetes mellitus presents to the emergency department with palpitations and dyspnea for 6 hours. His wife reports he has been increasingly fatigued over the past 2 weeks. On examination, pulse is irregularly irregular at 128/min, blood pressure 145/92 mmHg, and there are bilateral crackles at the lung bases. ECG shows absent P waves, irregular RR intervals, and a ventricular rate of 120–140 bpm. Chest X-ray shows pulmonary edema. Troponin I is normal. What is the most appropriate immediate management?